• Energy Research

The productivity of rainforests growing on highly weathered tropical soils is expected to be limited by phosphorus availability1. Yet, controlled fertilization experiments have been unable to demonstrate a dominant role for phosphorus in controlling tropical forest net primary productivity. Recent syntheses have demonstrated that responses to nitrogen addition are as large as to phosphorus2, and adaptations to low phosphorus availability appear to enable net primary productivity to be maintained across major soil phosphorus gradients3. Thus, the extent to which phosphorus availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterized by soils that are more depleted in phosphorus than those in which most tropical fertilization experiments have taken place2. Thus, we established a phosphorus, nitrogen and base cation addition experiment in an old growth Amazon rainforest, with a low soil phosphorus content that is representative of approximately 60% of the Amazon basin. Here we show that net primary productivity increased exclusively with phosphorus addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of phosphorus limitation of net primary productivity suggests that phosphorus availability may restrict Amazon forest responses to CO2 fertilization4, with major implications for future carbon sequestration and forest resilience to climate change.

Tree mortality appears to be increasing in moist tropical forests 1 , with potentially important implications for global carbon and water cycles 2 . Little is known about the drivers of tree mortality in these diverse forests, partly because long-term data are lacking 3 . The relative importance of climatic factors and species functional traits as drivers of tropical tree mortality are evaluated using a unique dataset in which the survival of over 1,000 rainforest canopy trees from over 200 species has been monitored monthly over five decades in the Central Amazon. We found that drought, as well as heat, storms and extreme rainy years, increase tree mortality for at least two years after the climatic event. Specific functional groups (pioneers, softwoods and evergreens) had especially high mortality during extreme years. These results suggest that predicted climate change will lead to higher tree mortality rates, especially for short-lived species, which may result in faster carbon sequestration but lower carbon storage of tropical forests.